Processing method for photoresist master, production method for recording medium-use mater, production method for recording medium, photoresist master, recording medium-use master and recording medium

ABSTRACT

The present invention relates to a method for processing a photoresist-coated board, a method for manufacturing a stamper for a recording medium and a method for manufacturing a recording medium which can form a fine raised and depressed pattern having a uniform width after development even in the case where a laser beam having a relatively long wavelength is used for forming pre-pits on a recording medium with high accuracy. A photoresist-coated board  108  is constituted by laminating a light absorption layer  108   b  and a photosensitive material layer  108   c  on a glass substrate  108   a  in this order and is exposed to a laser beam  102  by condensing the laser beam  102  onto the photosensitive material layer  108   c , thereby forming a raised and depressed pattern corresponding to pre-pits on the the photosensitive material layer  108 . When the length of a pre-pit to be formed is shorter than 4 T, for example, the duty ratio of the pulse signal train input to the light modulator  109  is varied within a range from about 50% to 65% so that a pulse signal train having a higher duty ratio is generated as the length of a pre-pit to be formed becomes longer and the power of a laser beam is modulated by the thus generated pulse signal train. On the other hand, when the length of a pre-pit to be formed is equal to or longer than 4 T, for example, a pulse signal train having a constant duty ratio is generated independently of the length of the pre-pit to be formed and the power of a laser beam is modulated by the thus generated pulse signal train.

BACKGROUND OF THE INVENTION

The present invention relates to a method for processing aphotoresist-coated glass board, a method for manufacturing a stamper fora recording medium, a method for manufacturing a recording medium, aphotoresist-coated glass board, a stamper for a recording medium and arecording medium and, particularly, to a method for processing aphotoresist-coated glass board to manufacture a stamper for a recordingmedium such as an optical recording disk accurately formed withpre-pits.

DESCRIPTION OF THE PRIOR ART

The kind of recording medium known as an “optical recording disk” ismanufactured by employing an optical recording disk stamper formed witha negative pattern of pits or guide grooves for tracking andtransferring the negative pattern of pits or guide grooves for trackingonto the optical recording disk by an injection molding process orphoto-setting process. The negative pattern of the stamper is formed byprojecting a laser beam onto a photo-sensitive layer formed by applyinga photo-sensitive material onto a glass substrate, thereby exposing thephoto-sensitive layer to the laser beam, applying a metal film made ofnickel or the like onto a raised and depressed pattern formed at regionsof the exposed photo-sensitive layer, effecting thick film plating usingthe metal film as a base, and peeling off the photo-sensitive layer fromthe metal film.

FIG. 7 is a diagram showing a process for manufacturing a stamper for anoptical recording disk using a cutting machine 100. As shown in FIG. 7,a laser beam 102 emitted from a laser device 101 enters an EOM(electro-optic modulator) 103, whereby the power of the laser beam 102is modulated to a predetermined power suitable for exposure and isreflected by a beam splitter 104, a beam splitter 105 and a mirror 106to be condensed by a lens 107 a onto a photoresist-coated board 108.When a stamper for a DVD is to be fabricated, a laser beam 102 having awavelength λ of about 351 nm is employed.

The photoresist-coated board 108 has a disk-like shape and includes aglass substrate 108 a and a photosensitive material layer 108 c formedon the glass substrate 108 a. The photosensitive material layer 108 c isirradiated with a laser beam 102 to be exposed thereto and developed toform a raised and depressed pattern composed of grooves or pre-pits.

A lens 107 b for condensing the laser 102, a light modulator 109 and alens 107 c for making the laser beam whose power has been modulated intoa parallel beam are disposed between the beam splitter 104 and the beamsplitter 105 and the power of the laser beam 102 is modulated inaccordance with an input pulse signal train. In such a case, the powerof the laser beam 102 is modulated intermittently in accordance with thelength of the pre-pits to be formed. For example, as disclosed inJapanese Patent Application Laid Open No. 2000-276780, the length of apulse for end portions of the pit is set longer than that for a centralportion of the pit and the length of a pulse following a foremost pulseand the length of a pulse preceding a last pulse are set longer thanthat at the central portion. Pre-pits each having a uniform width can beformed by modulating the power of the laser beam 102 in this manner,thereby improving jitter property and modulation of the reproducedsignal.

A latent image of a raised and depressed pattern corresponding topre-pits is formed spirally on the photoresist-coated board 108 byintegrally moving the mirror 106 and an objective lens 107 a in a radialdirection of the photoresist-coated board 108 and rotating a turn table110 on which the photoresist-coated board 108 is placed, and the raisedand depressed pattern is formed on the photoresist-coated board 108 bydeveloping the photoresist-coated board 108.

A nickel film is formed by plating and electrotyping on thephotoresist-coated board 108 on which the raised and depressed patterncorresponding to the pre-pits is formed in this manner and thephotoresist-coated board 108 is peeled off from the nickel film, therebyfabricating a stamper.

Recently, the recording density of data in an optical recording disk hasbeen increased. For example, although the track pitch of a DVD is set to0.74 μm and the shortest pit length thereof is set to 0.4 μm, in anext-generation type optical recording disk having an extremely highdata transfer rate, it is required to set the track pitch and theshortest pit length thereof to about 0.32 μm and 0.16 μm, respectively.In accordance with this trend, it is required to set the wavelength λ ofthe laser beam used for reproducing data from the optical recording diskto about 405 nm, i.e., shorter than the 650 nm wavelength of the laserbeam used for the conventional optical recording disk.

A stamper for producing such an optical recording disk is difficult tofabricate using a conventional cutting machine employing a laser beamhaving a wavelength λ of 351 nm. Specifically, in the case where astamper is fabricated using the conventional cutting machine employing alaser beam having a wavelength λ of 351 nm and only modulating the laserbeam to an intermittent beam, the width of the raised and depressedpatterns cannot be made uniform for forming raised and depressedpatterns having various widths corresponding to a short pit to a longpit, making it impossible to narrow the track pitch.

Although it is possible to form the pits of such an optical recordingdisk by using a cutting machine employing a laser beam having a shorterwavelength, such a cutting machine is expensive and it is desirable tofabricate a stamper for the next-generation type optical recording diskusing an existing cutting machine.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodfor processing a photoresist-coated board which can form a fine raisedand depressed pattern having a uniform width after development even inthe case where a laser beam having a relatively long wavelength is used,and a photoresist-coated board formed with a fine raised and depressedpattern having a uniform width.

It is another object of the present invention to provide a method formanufacturing a stamper for a recording medium such as an opticalrecording disk or the like which can make pit widths of pre-pits uniformirrespective of the pit lengths and narrow a track pitch even in thecase where a laser beam having a relatively long wavelength is used anda stamper for a recording medium such as an optical recording disk orthe like formed with pre-pits having uniform pit widths and a narrowtrack pitch.

It is a further object of the present invention to provide a method formanufacturing a recording medium such as an optical recording disk orthe like which can make pit widths of pre-pits uniform irrespective ofthe pit lengths and narrow a track pitch even in the case where a laserbeam having a relatively long wavelength is used and a recording mediumsuch as an optical recording disk or the like formed with pre-pitshaving uniform pit widths and a narrow track pitch.

The above objects of the present invention can be accomplished by amethod for processing a photoresist-coated board comprising steps ofintermittently projecting a pulse-like laser beam onto a photosensitivematerial layer on or above a light absorption layer provided in thephotoresist-coated board to expose the photosensitive material layer tothe laser beam, and developing the photosensitive material layer,thereby forming a raised and depressed pattern on the photosensitivematerial layer.

According to the present invention, since a laser beam is intermittentlyprojected onto a photoresist-coated board formed with a light absorptionlayer, a fine raised and depressed pattern having a uniform width can beformed on the photoresist-coated board after development even in thecase where a laser beam having a relatively long wavelength is used.Therefore, a stamper for a recording medium and a recording mediumformed with fine pre-pits having a uniform width can be fabricated.

In a preferred aspect of the present invention, a duty ratio of thepulse-like laser beam is varied in accordance with a length of adepressed portion or a projecting portion of the raised and depressedpattern to be formed.

According to this preferred aspect of the present invention, since thelaser beam is intermittently projected onto the photoresist-coated boardwhile the duty ratio thereof is varied, a fine raised and depressedpattern having a uniform width can be formed on the photoresist-coatedboard after development even in the case where a laser beam having arelatively long wavelength is used. Therefore, a stamper for a recordingmedium and a recording medium formed with fine pre-pits having a uniformwidth can be fabricated.

The above objects of the present invention can be also accomplished by amethod for processing a photoresist-coated board comprising steps ofintermittently projecting a pulse-like laser beam onto a photosensitivematerial layer provided in the photoresist-coated board to expose thephotosensitive material layer to the laser beam, and developing thephotosensitive material layer, thereby forming a raised and depressedpattern on the photosensitive material layer, wherein the method forprocessing a photoresist-coated board further comprises a step ofvarying a duty ratio of the pulse-like laser beam in accordance with alength of a depressed portion or a projecting portion of the raised anddepressed pattern to be formed.

According to the present invention, since the laser beam isintermittently projected onto the photoresist-coated board while theduty ratio thereof is varied, a fine raised and depressed pattern havinga uniform width can be formed on the photoresist-coated board afterdevelopment even in the case where a laser beam having a relatively longwavelength is used. Therefore, a stamper for a recording medium and arecording medium formed with fine pre-pits having a uniform width can befabricated.

In a preferred aspect of the present invention, when a length of adepressed portion or a projecting portion of the raised and depressedpattern is shorter than a predetermined length, the duty ratio of thepulse-like laser beam is varied in accordance with the length of thedepressed portion or the projecting portion of the raised and depressedpattern and when a length of a depressed portion or a projecting portionof the raised and depressed pattern is equal to or longer than apredetermined length, the duty ratio of the pulse-like laser beam iskept constant independently of the length of the depressed portion orthe projecting portion of the raised and depressed pattern, therebyexposing the photosensitive material layer to the laser beam.

According to this preferred aspect of the present invention, since theduty ratio of the pulse-like laser beam is varied only when it isparticularly difficult to form a fine raised and depressed patternhaving a uniform width without varying the duty ratio of the pulse-likelaser beam for forming a depressed portion or a projecting portion ofthe raised and depressed pattern having a length shorter than apredetermined length, it is possible to form the raised and depressedpattern on the photoresist-coated board so as to be fine and have auniform width without conducting complicated operations. Therefore, astamper for a recording medium and a recording medium formed with finepre-pits having a uniform width can be fabricated.

In a further preferred aspect of the present invention, the duty ratioof the pulse-like laser beam is varied so that the photosensitivematerial layer is exposed to the laser beam by setting the duty ratio ofthe pulse-like laser beam to a first value when a depressed portion or aprojecting portion of a raised and depressed pattern having a firstlength is to be formed and that the photosensitive material layer isexposed to the laser beam by setting the duty ratio of the pulse-likelaser beam to a second value smaller than the first value when adepressed portion or a projecting portion of a raised and depressedpattern having a second length longer than the first length is to beformed.

According to this preferred aspect of the present invention, since thelaser beam having a smaller duty ratio is intermittently projected ontothe photoresist-coated board when the length of the depressed portion orprojecting portion of the raised and depressed pattern becomes greater,a fine raised and depressed pattern having a uniform width can be formedon the photoresist-coated board after development irrespective of thelength of the depressed portion or projecting portion of the raised anddepressed pattern even in the case where a laser beam having arelatively long wavelength is used. Therefore, a stamper for a recordingmedium and a recording medium formed with fine pre-pits having a uniformwidth can be fabricated.

The above objects of the present invention can be also accomplished by amethod for manufacturing a stamper for a recording medium comprising astep of intermittently projecting a pulse-like laser beam onto aphotosensitive material layer on or above a light absorption layerprovided in the photoresist-coated board to expose the photosensitivematerial layer to the laser beam, and developing the photosensitivematerial layer, thereby forming a raised and depressed pattern on thephotosensitive material layer and a step for transferring the raised anddepressed pattern formed on the photosensitive material layer onto thestamper for a recording medium, thereby forming a raised and depressedpattern on the stamper for an optical recording medium.

According to the present invention, since a laser beam is intermittentlyprojected onto a photoresist-coated board formed with a light absorptionlayer, a fine raised and depressed pattern having a uniform width can beformed on the photoresist-coated board after development even in thecase where a laser beam having a relatively long wavelength is used.Therefore, a recording medium formed with fine pre-pits having a uniformwidth can be fabricated.

The above objects of the present invention can be also accomplished by amethod for manufacturing a stamper for a recording medium comprising astep of forming a raised and depressed pattern including a depressedportion or a projecting portion corresponding to a pit on aphotosensitive material layer on or above a light absorption layerprovided in a photoresist-coated board by projecting two or more pulsesof a laser beam onto the photosensitive material layer to expose thephotosensitive material layer thereto and developing the photosensitivematerial layer, and a step of transferring the raised and depressedpattern formed on the photosensitive material layer onto a stamper for arecording medium and forming a raised and depressed pattern on thestamper. Here, “two pulses” means two intermittent pulses.

According to the present invention, since two or more pulses of a laserbeam are projected onto the photoresist-coated board having the lightabsorption layer, a fine raised and depressed pattern having a uniformwidth can be formed on the photoresist-coated board after developmenteven in the case where a laser beam having a relatively long wavelengthis used. Therefore, a recording medium formed with fine pre-pits havinga uniform width can be fabricated.

In a preferred aspect of the present invention, a duty ratio of thepulse-like laser beam is varied in accordance with a length of adepressed portion or a projecting portion of the raised and depressedpattern to be formed.

According to this preferred aspect of the present invention, since thelaser beam is intermittently projected onto the photoresist-coated boardwhile the duty ratio thereof is varied, a fine raised and depressedpattern having a uniform width can be formed on the photoresist-coatedboard after development even in the case where a laser beam having arelatively long wavelength is used. Therefore, a recording medium formedwith fine pre-pits having a uniform width can be fabricated.

The above objects of the present invention can be also accomplished by amethod for manufacturing a stamper for a recording medium comprising astep of forming a raised and depressed pattern on a photosensitivematerial layer provided in a photoresist-coated board by intermittentlyprojecting a pulse-like laser beam onto the photosensitive materiallayer to expose the photosensitive material layer thereto and developingthe photosensitive material layer, and a step of transferring the raisedand depressed pattern formed on the photosensitive material layer onto astamper for a recording medium and forming a raised and depressedpattern on the stamper, wherein the method for manufacturing a stamperfor a recording medium further comprises a step of varying a duty ratioof the pulse-like laser beam in accordance with a length of a depressedportion or a projecting portion of the raised and depressed pattern tobe formed.

According to the present invention, since the laser beam isintermittently projected onto the photoresist-coated board while theduty ratio thereof is varied, a fine raised and depressed pattern havinga uniform width can be formed on the photoresist-coated board afterdevelopment even in the case where a laser beam having a relatively longwavelength is used. Therefore, a recording medium formed with finepre-pits having a uniform width can be fabricated.

In a preferred aspect of the present invention, when a length of adepressed portion or a projecting portion of the raised and depressedpattern is shorter than a predetermined length, the duty ratio of thepulse-like laser beam is varied in accordance with the length of thedepressed portion or the projecting portion of the raised and depressedpattern and when a length of a depressed portion or a projecting portionof the raised and depressed pattern is equal to or longer than apredetermined length, the duty ratio of the pulse-like laser beam iskept constant independently of the length of the depressed portion orthe projecting portion of the raised and depressed pattern, therebyexposing the photosensitive material layer to the laser beam.

According to this preferred aspect of the present invention, since theduty ratio of the pulse-like laser beam is varied only when it isparticularly difficult to form a fine raised and depressed patternhaving a uniform width without varying the duty ratio of the pulse-likelaser beam for forming a depressed portion or a projecting portion ofthe raised and depressed pattern having a length shorter than apredetermined length, it is possible to form the raised and depressedpattern on the photoresist-coated board so as to be fine and have auniform width without conducting complicated operations. Therefore, arecording medium formed with fine pre-pits having a uniform width can befabricated.

In a further preferred aspect of the present invention, the duty ratioof the pulse-like laser beam is varied so that the photosensitivematerial layer is exposed to the laser beam by setting the duty ratio ofthe pulse-like laser beam to a first value when a depressed portion or aprojecting portion of a raised and depressed pattern having a firstlength is to be formed and that the photosensitive material layer isexposed to the laser beam by setting the duty ratio of the pulse-likelaser beam to a second value smaller than the first value when adepressed portion or a projecting portion of a raised and depressedpattern having a second length longer than the first length is to beformed.

According to this preferred aspect of the present invention, since thelaser beam having a smaller duty ratio is intermittently projected ontothe photoresist-coated board when the length of the depressed portion orprojecting portion of the raised and depressed pattern becomes greater,a fine raised and depressed pattern having a uniform width can be formedon the photoresist-coated board after development irrespective of thelength of the depressed portion or projecting portion of a raised anddepressed pattern even in the case where a laser beam having arelatively long wavelength is used. Therefore, a recording medium formedwith fine pre-pits having a uniform width can be fabricated.

The above objects of the present invention can be also accomplished by amethod for manufacturing a recording medium comprising at least a stepof forming a raised and depressed pattern including a depressed portionor a projecting portion corresponding to a pit on a photosensitivematerial layer on or above a light absorption layer provided in aphotoresist-coated board by projecting two or more pulses of a laserbeam onto the photosensitive material layer to expose the photosensitivematerial layer thereto and developing the photosensitive material layer,a step of transferring the raised and depressed pattern formed on thephotosensitive material layer onto a stamper for a recording medium andforming a raised and depressed pattern on the stamper, and a step oftransferring the raised and depressed pattern formed on the stamper ontoa substrate for a recording medium to form pre-pits on the substrate.

According to the present invention, since two or more pulses of a laserbeam are projected onto the photoresist-coated board having the lightabsorption layer, a fine raised and depressed pattern having a uniformwidth can be formed on the photoresist-coated board after developmenteven in the case where a laser beam having a relatively long wavelengthis used.

According to this preferred aspect of the present invention, since thelaser beam is intermittently projected onto the photoresist-coated boardwhile the duty ratio thereof is varied, a fine raised and depressedpattern having a uniform width can be formed on the photoresist-coatedboard after development even in the case where a laser beam having arelatively long wavelength is used.

The above objects of the present invention can be also accomplished by amethod for manufacturing a recording medium comprising at least a stepof forming a raised and depressed pattern including a depressed portionor a projecting portion corresponding to a pit on a photosensitivematerial layer provided in a photoresist-coated board by projecting twoor more pulses of a laser beam onto the photosensitive material layer toexpose the photosensitive material layer thereto and developing thephotosensitive material layer, a step of transferring the raised anddepressed pattern formed on the photosensitive material layer onto astamper for an optical recording medium and forming a raised anddepressed pattern on the stamper, and a step of transferring the raisedand depressed pattern formed on the stamper onto a substrate for arecording medium to form pre-pits on the substrate, wherein the methodfor manufacturing a recording medium further comprises a step of varyinga duty ratio of the pulse-like laser beam in accordance with a length ofa pre-pit to be formed.

According to the present invention, since the laser beam isintermittently projected onto the photoresist-coated board while theduty ratio thereof is varied, a fine raised and depressed pattern havinga uniform width can be formed on the photoresist-coated board afterdevelopment even in the case where a laser beam having a relatively longwavelength is used.

In a preferred aspect of the present invention, when a length of apre-pit is shorter than a predetermined length, the duty ratio of thepulse-like laser beam is varied in accordance with the length of thepre-pit and when a length of a pre-pit is equal to or longer than apredetermined length, the duty ratio of the pulse-like laser beam iskept constant independently of the length of the pre-pit, therebyexposing the photosensitive material layer to the laser beam.

According to this preferred aspect of the present invention, since theduty ratio of the pulse-like laser beam is varied only when it isparticularly difficult to form a fine raised and depressed patternhaving a uniform width without varying the duty ratio of the pulse-likelaser beam for forming a depressed portion or a projecting portion ofthe raised and depressed pattern having a length shorter than apredetermined length, it is possible to form the raised and depressedpattern on the photoresist-coated board so as to be fine and have auniform width without conducting complicated operations.

In a further preferred aspect of the present invention, the duty ratioof the pulse-like laser beam is varied so that the photosensitivematerial layer is exposed to the laser beam by setting the duty ratio ofthe pulse-like laser beam to a first value when a pre-pit having a firstlength is to be formed and that the photosensitive material layer isexposed to the laser beam by setting the duty ratio of the pulse-likelaser beam to a second value smaller than the first value when a pre-pithaving a second length longer than the first length is to be formed.

According to this preferred aspect of the present invention, since thelaser beam having a smaller duty ratio is intermittently projected ontothe photoresist-coated board when the length of the depressed portion orprojecting portion of the raised and depressed pattern becomes greater,a fine raised and depressed pattern having a uniform width can be formedon the photoresist-coated board after development irrespective of thelength of the depressed portion or projecting portion of a raised anddepressed pattern even in the case where a laser beam having arelatively long wavelength is used.

The above objects of the present invention can be also accomplished by aphotoresist-coated board constituted by laminating a light absorptionlayer and a photosensitive material layer on a substrate in this order,the photosensitive material layer being formed with a raised anddepressed pattern and the raised and depressed pattern of thephotosensitive material layer being formed by intermittently projectinga pulse-like laser beam onto the photosensitive material layer to exposethe photosensitive material layer thereto and developing thephotosensitive material layer.

According to the present invention, since the raised and depressedpattern on the photoresist-coated board is fine and has a uniform width,it is possible to manufacture a stamper for a recording medium and arecording medium formed with a fine raised and depressed pattern havinga uniform width using the photoresist-coated board.

In a preferred aspect of the present invention, the raised and depressedpattern of the photosensitive material layer is formed by intermittentlyprojecting on the photosensitive material layer the pulse-like laserbeam whose duty ratio is varied in accordance with a length of adepressed portion or a projecting portion of the raised and depressedpattern and developing the photosensitive material layer.

According to this preferred aspect of the present invention, since theraised and depressed pattern on the photoresist-coated board is fine andhas a uniform width, it is possible to manufacture a stamper for arecording medium and a recording medium formed with a fine raised anddepressed pattern having a uniform width using the photoresist-coatedboard.

The above objects of the present invention can be also accomplished by aphotoresist-coated board constituted by laminating a photosensitivematerial layer on a substrate, the photosensitive material layer beingformed with a raised and depressed pattern and the raised and depressedpattern of the photosensitive material layer being formed byintermittently projecting a pulse-like laser beam onto thephotosensitive material layer to expose the photosensitive materiallayer thereto and developing the photosensitive material layer.

According to the present invention, since the raised and depressedpattern on the photoresist-coated board is fine and has a uniform width,it is possible to manufacture a stamper for a recording medium and arecording medium formed with a fine raised and depressed pattern havinga uniform width using the photoresist-coated board.

The above objects of the present invention can be also accomplished by astamper for a recording medium having a raised and depressed patternformed by transferring the raised and depressed pattern of one of theaforesaid photoresist-coated boards thereonto.

According to this preferred aspect of the present invention, since thelaser beam is intermittently projected onto the photoresist-coatedboard, a fine raised and depressed pattern having a uniform width can beformed in the photoresist-coated board after development even in thecase where a laser beam having a relatively long wavelength is used.Therefore, a recording medium formed with fine pre-pits having a uniformwidth can be fabricated.

The above objects of the present invention can be also accomplished by arecording medium having pre-pits formed by transferring the raised anddepressed pattern of the stamper for a recording medium thereonto.

According to the present invention, it is possible to make a pit-widthof the pre-pits formed on the recording medium uniform and narrow atrack pitch.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the principle of a method for manufacturinga stamper for an optical recording disk which is a preferred embodimentof the present invention.

FIGS. 2( a) to 2(f) show steps for manufacturing a stamper for anoptical recording disk.

FIGS. 3( a)-3(c) are diagrams showing a pulse signal train input to alight modulator 109 and a plan view of a raised and depressed pattern202 formed thereby on a photoresist-coated glass board 108.

FIGS. 4( a)-(c) are diagrams showing a method for generating a pulsesignal train that is a signal for modulating the power of a laser beam.

FIG. 5 is a block diagram of one example of a pulse signal generationcircuit.

FIGS. 6( a) to 6(c) show steps for manufacturing an optical recordingdisk.

FIG. 7 is a diagram showing a method for manufacturing a stamper for anoptical recording disk using a cutting machine 100.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will now be describedwith reference to the accompanying drawings.

FIG. 1 is a diagram showing the principle of a method for manufacturinga stamper for an optical recording disk which is a preferred embodimentof the present invention. As shown in FIG. 1, a laser beam 102 emittedfrom a laser beam source 101 such as a krypton laser beam source entersan EOM (electro-optic modulator) 103, whereby the power thereof ismodulated to a predetermined power suitable for exposure and isreflected by a beam splitter 104, a beam splitter 105 and a mirror 106to be condensed by a lens 107 a onto a photoresist-coated glass board108.

The photoresist-coated glass board 108 has a disk-like shape and isconstituted by laminating a light absorption layer 108 b and aphotosensitive material layer 108 c on a glass substrate 108 a in thisorder and the laser beam 102 is condensed onto the photosensitivematerial layer 108 c, whereby the photosensitive material layer 108 c isexposed to the laser beam 102 and a latent image of a raised anddepressed pattern corresponding to pre-pits is formed therein.

The light absorption layer 108 b preferably contains an organic compoundhaving a light absorbency (sometimes referred to as “a light absorbentagent”). It is preferable to use at least one kind of a compoundselected from a photo-initiator and a dye as the light absorbent agent.Generally, a photo-initiator is used together with a photo-curable resinand has a property of generating a radical upon absorbing light such asan ultraviolet ray. Further, it is preferable to incorporate aphoto-initiation auxiliary agent that itself does not become activatedbut serves to promote photo-chemical reaction when used together withthe photo-initiator, as compared with the case where the photo-initiatoris used alone, whereby the photo-chemical reaction efficiently proceeds.The photo-initiator is decomposed when it generates a radical but thephoto-initiation auxiliary agent remains stable. It is thereforepreferable to use a photo-initiation auxiliary agent in the presentinvention.

As a photo-initiation auxiliary agent, aliphatic amine or aromatic amineis mainly used. In the present invention, it is preferable to employ asa photo-initiation auxiliary agent at least one kind of a compoundselected from 4, 4′-bis (dimethylamino) benzophenone, 4, 4′-bis(diethylamino) benzophenone, 4-dimethylamino amyl benzoate,4-dimethylamino (n-butoxy) ethyl benzoate, 4-dimethylamino isoamylbenzoate and 4-dimethylamino 2-ethylhexyl benzoate. Among these, it ispreferable to employ a benzophenone system compound.

It is preferable to form a light absorption layer containing a lightabsorbent agent in the following manner. A light absorbent agent isfirst dissolved in a solvent to prepare a coating solution. Athermo-crosslinking compound may be added to the coating solutiontogether with the light absorbent agent as occasion demands. If acoating layer containing the thermo-crosslinking compound in addition tothe light absorbent agent is formed, the coating layer is heated to behardened and a photo-resist layer is formed on the thus hardened coatinglayer, it is possible to prevent the light absorption layer and thephoto-resist layer from mixing. Further, various additives such as anadhesion auxiliary agent for improving adhesiveness with thephoto-resist layer, a light absorbance agent, a surface activesurfactant and the like may be added to the coating solution as occasiondemands.

The content of the light absorbent agent in the light absorption layeris preferably 10 to 70 weight %. When the content of the light absorbentagent is too small, it is difficult to achieve a sufficient lightabsorbance. On the other hand, when the content of the light absorbentagent is too large, the content of the material produced by thehardening of thermo-crosslinking compound becomes small, so that a lightabsorption layer having sufficient strength cannot be formed. The lightabsorption layer preferably has an absorption (extinction) coefficient kequal to or higher than 0.01 in the range of the wavelength of the laserbeam used for fabricating the stamper and more preferably has anabsorption (extinction) coefficient k equal to or higher than 0.1. Whenthe absorption coefficient k is too small, the laser beam cannot besufficiently absorbed by the light absorption layer.

The thickness of the light absorption layer is not particularly limitedand the light absorption layer is formed so as to have such a thicknessthat the laser beam used for exposing the photoresist layer can besufficiently absorbed thereby. When the light absorption layer is toothin, it cannot absorb a sufficient amount of the laser beam and thephotoresist layer tends to be multiply exposed, whereby a latent imagehaving a desired shape cannot be formed. On the other hand, formation ofthe light absorption layer to a thickness larger than 300 nm does notmarkedly improve the light absorbance property, so that it isunnecessary to form the light absorption layer to a thickness largerthan 300 nm. Further, when the light absorption layer is formed so as tohave a thickness larger than 300 nm, excessive heat is accumulated inthe light absorption layer when a laser beam is projected thereonto,whereby the photoresist layer tends to be thermally decomposed so thatit becomes difficult to stably expose the photoresist layer. Therefore,the thickness of the light absorption layer is preferably 1 nm to 300 nmand more preferably 10 nm to 200 nm. However, since the degree of thethermal decomposition of the photoresist layer caused by the heataccumulation in the light absorption layer varies depending upon thepower of the laser beam projected onto the light absorption layer, inthe case where the photoresist layer is exposed to a laser beam havingrelatively low power, it is possible to form the light absorption layerso as to have a thickness larger than 300 nm and equal to or smallerthan 500 nm, for example.

A lens 107 b for condensing the laser 102, a light modulator 109 and alens 107 c for making the laser beam whose power has been modulated intoa parallel beam are disposed between the beam splitter 104 and the beamsplitter 105 and the power of the laser beam 102 is modulated inaccordance with an input pulse signal train. Therefore, the laser beamis modulated into intermittent pulses having a predetermined duty ratio.Here, the duty ratio is defined as the ratio of the pulse width of thepulse-like laser beam (pulse laser) to the cycle of repetition ofpulses.

A latent image of pre-pits 110 is formed spirally on thephotoresist-coated glass board 108 by integrally moving the mirror 106and an objective lens 107 a in a radial direction of thephotoresist-coated glass board 108 and rotating a turn table 110 onwhich the photoresist-coated glass board 108 is placed.

FIGS. 2( a) to 2(f) show steps for manufacturing a stamper for anoptical recording disk. The photoresist-coated glass board 108 includesthe glass substrate 108 a, the light absorption layer 108 b formed onthe glass substrate 108 a and the photosensitive material layer 108 cformed on the light absorption layer 108 b. Although not shown inFigures, the photoresist-coated glass board 108 may include an adhesivelayer (primer) for improving adhesiveness between the glass substrate108 a and the light absorption layer 108 b (FIG. 2( a)).

When the laser beam 102 is condensed by the lens 107 a onto thephotosensitive material layer 108 c of the photoresist-coated glassboard 108, a region of the photosensitive material layer 108 cirradiated with the laser beam 102 is exposed to the laser beam 102(FIG. 2( b)). Thereby, a latent image of a raised and depressed patterncorresponding to a pre-pit is formed. At this time, the power of thelaser beam 102 is modulated by the light modulator 109. After theexposure of the photoresist-coated glass board 108, a developingsolution such as a sodium hydroxide solution is sprayed on thephotoresist-coated glass board 108, whereby the raised and depressedpattern 202 is developed (FIG. 2( c)).

After the photoresist-coated glass board 108 has been exposed to thelaser beam 102 and developed, a thin nickel film 203 is formed on thephotosensitive material layer 108 c using an electroless depositionprocess or a vapor deposition process (FIG. 2( d)). Thereafter, a thicknickel film 204 having a thickness of about 0.3 mm is formed by thickplating using the thin nickel film 203 as a positive electrode andnickel as a negative electrode (FIG. 2( e)).

After forming the thick nickel film 204, the resist surface is peeledoff from the thin nickel film 203, a laminate of the thin nickel film203 and the thick nickel film 204 is washed and processed to desiredinner and outer diameters, thereby fabricating a stamper 205 for anoptical recording disk (FIG. 2( f)). Optical recording disks can bemass-produced by using the thus fabricated stamper 205 and transferringthe raised and depressed pattern on the optical recording disk by aninjection molding process, a 2P process or the like.

Here, the number of kinds of the raised and depressed pattern 202 isdetermined by the modulation code used for the optical recording disk tobe manufactured. While other possibilities also exist, in the case wherea (1, 7)RLL Modulation Code is used for a next-generation type opticalrecording disk having an extremely high data transfer rate, sincepre-pits having lengths of 2 T to 8 T are used, the number of kinds ofthe raised and depressed pattern 202 is seven, i.e., 2 T to 8 T.

Each of FIG. 3( a) to 3(c) is a diagram showing a pulse signal traininput to a light modulator 109 and a plan view of a depressed portion ora projecting portion 202 of a raised and depressed pattern formedthereby on a photoresist-coated glass board 108. As shown in FIG. 3, inthe case where a depressed portion or a projecting portion of a raisedand depressed pattern corresponding to pre-pits having lengths of 2 T to8 T is to be formed, when the length of a pre-pit to be formed isshorter than 4 T, for example, the duty ratio of the pulse signal traininput to the light modulator 109 is varied within a range from about 30%to 80%, preferably, about 50% to 65% so that a pulse signal train havinga higher duty ratio is generated as the length of a pre-pit to be formedbecomes longer and the power of a laser beam is modulated by the thusgenerated pulse signal train. On the other hand, when the length of apre-pit to be formed is equal to or longer than 4 T, for example, it ispreferable to generate a pulse signal train having a constant duty ratioindependently of the length of the pre-pit to be formed and modulate thepower of the laser beam using the thus generated pulse signal train.

For example, in the case where a pre-pit having a length of 2 T is to beformed, pulse signals 301 of two clock cycles having a duty ratio ofabout 65% are used and a depressed portion or a projecting portion 302of a raised and depressed pattern corresponding to a pre-pit having alength of 2 T is formed (FIG. 3( a)).

Further, in the case where a pre-pit having a length of 3 T is to beformed, pulse signals 303 of three clock cycles having a duty ratio ofabout 60% are used and a depressed portion or a projecting portion 304of a raised and depressed pattern corresponding to a pre-pit having alength of 3 T is formed (FIG. 3( b)).

Furthermore, in the case where a pre-pit having a length of one of 4 Tto 8 T is to be formed, the duty ratio of a pulse signal 305 is set toabout 50% so as to form a depressed portion or a projecting portion 306of a raised and depressed pattern corresponding to a pre-pit having alength of 4 T using the pulse signals 305 of four clock cycles, form adepressed portion or a projecting portion 306 of a raised and depressedpattern corresponding to a pre-pit having a length of 5 T using thepulse signals 305 of five clock cycles, form a raised and depressedpattern 306 corresponding to a pre-pit having a length of 6 T using thepulse signals 305 of six clock cycles, form a depressed portion or aprojecting portion 306 of a raised and depressed pattern correspondingto a pre-pit having a length of 7 T using the pulse signals 305 of sevenclock cycles and form a depressed portion or a projecting portion 306 ofa raised and depressed pattern corresponding to a pre-pit having alength of 8 T using the pulse signals 305 of eight clock cycles (FIG. 3(c)).

Thus, if the power of the laser beam is modulated using a pulse signaltrain having a proper duty ratio and the thus power-modulated laser beamis projected onto the photoresist-coated glass board 108, it is possibleeven when using a laser beam having a relatively long wavelength tonarrow the pit width w of a depressed portion or a projecting portion302, 304, 305 of a raised and depressed pattern corresponding to apre-pit and make it uniform independently of the length of the pre-pitand, as a result, the track pitch can be narrowed.

Here, the critical length of a pre-pit based on which it is determinedwhether the duty ratio of the pulse signal train should be varied or theduty ratio of the pulse signal train should be maintained constant ispreferably selected so as to be substantially equal to the size of thespot of the laser beam. In the present invention, the critical length ofa pre-pit is preferably determined to be 4 T but it may be determined tobe 2 T, 3 T or 5 T.

Further, the above mentioned duty ratios are only illustrated asexamples and it is possible to set the duty ratio of each of the pulsesignals to 60% when a pre-pit having a length of 2 T is to be formed,set the duty ratio of each of the pulse signals to 55% when a pre-pithaving a length of 3 T is to be formed and set the duty ratio of each ofthe pulse signals to 55% when a pre-pit having a length of one of 4 T to8 T is to be formed. Insofar as the duty ratio of each pulse signal isset in the range of 50% to 65% in this manner, substantially the sameadvantages can be obtained. However, when a pre-pit having a length ofone of 4 T to 8 T is to be formed, it is not preferable to set the dutyratio of each pulse signal too high, for example, about 60%.Furthermore, it is possible to vary the duty ratio of each pulse signalwithin the range of 30% to 80%, for example, in such a manner that theduty ratio of each pulse signal is set to 80% when a pre-pit having alength of 2 T is to be formed, set to 60% when a pre-pit having a lengthof 3 T is to be formed and set to 30% when a pre-pit having a length ofone of 4 T to 8 T is to be formed.

Moreover, even in the case where a pre-pit having a length equal to orlonger than a predetermined length, for example, 4 T, is to be formed,it is not absolutely necessary to maintain the duty ratio of each pulsesignal constant. Even in such a case, it is possible to form a pre-pithaving a length of one of 2 T to 8 T by stepwise varying the duty ratioof each pulse signal within the range of 50% to 65%, for example, insuch a manner that the duty ratio of each pulse signal is set to 65%when a pre-pit having a length of 2 T is to be formed, set to 60% when apre-pit having a length of 3 T is to be formed, set to 55% when apre-pit having a length of 4 T is to be formed, set to 53% when apre-pit having a length of 5 T is to be formed, set to 52% when apre-pit having a length of 6 T is to be formed, set to 51% when apre-pit having a length of 7 T is to be formed and set to 50% when apre-pit having a length of 8 T is to be formed.

Furthermore, it is possible to form a pre-pit having a length of one of2 T to 8 T by stepwise varying the duty ratio of each pulse signalwithin the range of 30% to 80%, for example, in such a manner that theduty ratio of each pulse signal is set to 80% when a pre-pit having alength of 2 T is to be formed, set to 65% when a pre-pit having a lengthof 3 T is to be formed, set to 50% when a pre-pit having a length of 4 Tis to be formed, set to 45% when a pre-pit having a length of 5 T is tobe formed, set to 40% when a pre-pit having a length of 6 T is to beformed, set to 35% when a pre-pit having a length of 7 T is to be formedand set to 30% when a pre-pit having a length of 8 T is to be formed.

FIG. 4 is a diagram showing a method for generating a pulse signal trainthat is a signal for modulating the power of a laser beam. A pulsesignal having a predetermined duty ratio (FIG. 4( c)) can be generatedby adding a rectangular wave signal of a base clock cycle having afrequency f of 20 MHz (FIG. 2( a)) and rectangular wave signal (FIG. 4(b)) obtained by phase shifting the base clock cycle. For example, apulse signal having a duty ratio of 65% can be generated by shifting thebase clock cycle by 3π/10 and adding the thus obtained rectangular waveto the rectangular wave of the base clock cycle. The base clock cycle isshifted by π/5 for generating a pulse signal having a duty ratio of 60%and is shifted by π/10 for generating a pulse signal having a duty ratioof 55%.

FIG. 5 is a block diagram of one example of a pulse signal generationcircuit. As shown in FIG. 5, the pulse signal generation circuitincludes a phase shifter 501 and a summer 502. The base clock cycle isdivided into two base cycles at a bifurcating point 503 and one of thebase clock cycles is input to the phase shifter 501. The base clockcycle is shifted by the phase shifter 501 by a predetermined shiftamount and output from the phase shifter 501. The shift amount of thebase clock cycle is determined based on pit length data input to thephase shifter 501. The summer 502 receives the original base clock cycleand the thus shifted base clock cycle and adds them to output the sumbase clock cycle.

A method for manufacturing an optical recording disk using the thusfabricated stamper will be described later.

FIGS. 6( a) to 6(c) show steps for manufacturing an optical recordingdisk. In a next-generation type optical recording disk having anextremely high data transfer rate, it is necessary to form a lighttransmission layer that is very much thinner than that of a CD or a DVD.

An optical recording disk substrate 601 having a thickness of about 1.1mm is first fabricated by an injection molding process using the stamper205 fabricated in the above described manner. Thereby obtaining anoptical recording disk substrate 601 onto which the raised and depressedpattern on the surface of the stamper 205 has been transferred (FIG. 6(a)). The material for forming the optical recording disk substrate 601is not particularly limited and polycarbonate, for example, may be usedfor forming the optical recording disk substrate 601. Further, theoptical recording disk substrate 601 may be fabricated by aphoto-setting process (2P process) using the stamper 205.

Next, as occasion demands, a sputtering apparatus is used to sputtermetal onto the surface of the optical recording disk substrate 601formed with the raised and depressed pattern. Thereby, a reflective film602 is formed on the surface of the optical recording disk substrate 601(FIG. 6( b)). It is preferable to form the reflective film 602 so as tohave a thickness of 10 nm to 300 nm and to employ an alloy containing Agas a primary component as the metal for forming the reflective film 602.

Then, ultraviolet ray curable resin is applied using a spin coatingmethod, a roll coating method, a screen printing method or the like ontothe surface of the optical recording disk substrate 601 formed with theraised and depressed pattern or the surface of the reflective film 602and an ultraviolet ray is projected onto the ultraviolet ray curableresin to form a light transmission layer 603 having a thickness of about10 μm to 300 μm (FIG. 6( c)). This completes the fabrication of theoptical recording disk. Here, the light transmission layer 603 may beformed by adhering a resin sheet formed of polycarbonate, polyolefin orthe like in advance onto the surface of the reflective film 602.

Further, a moisture-proof layer for preventing the reflective film 602from being corroded may be formed between the optical recording disksubstrate 601 and the reflective film 602 and/or the reflective film 602and the light transmission layer 603.

As described above, according to this embodiment, since a pulse-likelaser beam is intermittently projected onto the photoresist-coated glassboard having the light absorption layer, it is possible even when alaser beam having a relatively long wavelength is used to form pre-pitsso as to have a uniform width irrespective of the lengths of thepre-pits and, as a result, the track pitch can be narrowed. Therefore,it is possible to manufacture a stamper for an optical recording diskformed with pre-pits having a uniform pit width and a narrow trackpitch.

More specifically, according to this embodiment, it is possible tomanufacture a stamper for a next-generation type optical recording diskformed with pre-pits having track pitch of 0.32 μm and a shortest pitlength of about 0.16 μm using a laser beam having a wavelength of 351 nmused for manufacturing a stamper for a DVD.

Further, according to this embodiment, since the power of the laser beamis modulated based on a pulse signal train having a proper duty ratioand the thus power-modulated pulse-like laser beam is intermittentlyprojected onto the photoresist-coated glass board, even when a laserbeam having a relatively long wavelength is used, pre-pits can be formedso as to have a uniform width irrespective of the lengths of thepre-pits and, as a result, the track pitch can be narrowed.

The present invention has thus been shown and described with referenceto a specific embodiment. However, it should be noted that the presentinvention is in no way limited to the details of the describedarrangements but changes and modifications may be made without departingfrom the scope of the appended claims.

For example, although in the above described embodiment the laser beamis intermittently projected onto the photoresist-coated glass boardhaving the light absorption layer, it is not absolutely necessary tointermittently project the laser beam onto the photoresist-coated glassboard having the light absorption layer and the laser beam may beintermittently projected onto a photoresist-coated glass board having nolight absorption layer. Specifically, in the case where the power of thelaser beam is modulated based on a pulse signal train having a properduty ratio and the thus power-modulated pulse-like laser beam isintermittently projected onto a photoresist-coated glass board having nolight absorption layer, pre-pits cannot be formed so accurately as inthe case of employing the photoresist-coated glass board having thelight absorption layer but it is nevertheless possible to make the pitwidths of pre-pits sufficiently uniform and the track pitch considerablynarrow as compared with the case where the power of a laser beam ismodulated by a pulse signal train having pulse lengths determined basedon the lengths of pre-pits to be formed and the thus power-modulatedlaser beam is projected onto the photoresist-coated glass board.

Further, in the above described embodiment, for example, althoughexplanation was made as to the case where a stamper for an opticalrecording disk is fabricated using a photoresist-coated glass board, thepresent invention is not limited to such a case but can be widelyapplied for forming a fine raised and depressed pattern having a uniformwidth on a photoresist-coated glass board using a laser beam having arelatively long wavelength.

Furthermore, in the above described embodiment, for example, althoughthe photoresist-coated glass board and the stamper for an opticalrecording disk are shaped to be disk-like, it is not absolutelynecessary for the photoresist-coated glass board and the stamper for anoptical recording disk to have disk-like shapes and thephotoresist-coated glass board and the stamper for the optical recordingdisk may be shaped to be card-like in, for example, a rectangular form.

Moreover, in the above described embodiment, although explanation wasmade as to an optical recording disk, the present invention is notlimited to an optical recording disk but can be widely applied torecording media including a magnetic disk that is, for example, adiscrete medium.

Further, in the above described embodiment, for example, although theduty ratio of the laser pulse is controlled by shifting the base clockcycle using the pulse generation circuit including the phase shifter andthe summer, it is not absolutely necessary to control the duty ratio ofthe laser pulse by shifting the base clock cycle using the pulsegeneration circuit including the phase shifter and the summer, and theduty ratio of the laser pulse may be controlled using any of variousmethods.

As described above, according to the present invention, it is possibleto provide a method for processing a photoresist-coated board and aphotoresist-coated board in which a fine raised and depressed patternhaving a uniform width can be formed after development even in the casewhere a laser beam having a relatively long wavelength is used.

Therefore, according to the present invention, it is possible tomanufacture a stamper for a next-generation type optical recording diskhaving a track pitch of 0.32 μm and a shortest pit length of about 0.16μm using a cutting machine used for manufacturing a stamper for aconventional optical recording disk.

Further, according to the present invention, it is possible to provide amethod for manufacturing a stamper for a recording medium which can makepit widths of pre-pits uniform irrespective of the pit lengths and thetrack pitch narrow even in the case where a laser beam having arelatively long wavelength is used, and a stamper for an opticalrecording medium formed with pre-pits having uniform and a narrow trackpitch.

Furthermore, according to the present invention, it is possible toprovide a method for manufacturing a recording medium which can make pitwidths of pre-pits uniform irrespective of the pit lengths and the trackpitch narrow even in the case where a laser beam having a relativelylong wavelength is used, and a recording medium formed with pre-pitshaving uniform and a narrow track pitch.

1. A method for processing a photoresist-coated board comprising stepsof intermittently projecting a pulse-like laser beam onto aphotosensitive material layer on or above a light absorption layerprovided in the photoresist-coated board to expose the photosensitivematerial layer to the laser beam, and developing the photosensitivematerial layer, thereby forming a raised and depressed pattern on thephotosensitive material layer, wherein when a length of a depressedportion or a projecting portion of the raised and depressed pattern isshorter than a predetermined length, a duty ratio of the pulse-likelaser beam is varied in accordance with the length of the depressedportion or the projecting portion of the raised and depressed pattern,and when the length of the depressed portion or the projecting portionof the raised and depressed pattern is equal to or longer than thepredetermined length, the duty ratio of the pulse-like laser beam iskept constant independently of the length of the depressed portion orthe projecting portion of the raised and depressed pattern, therebyexposing the photosensitive material layer to the laser beam.
 2. Amethod for processing a photoresist-coated board comprising steps ofintermittently projecting a pulse-like laser beam onto a photosensitivematerial layer provided in the photoresist-coated board to expose thephotosensitive material layer to the laser beam, and developing thephotosensitive material layer, thereby forming a raised and depressedpattern on the photosensitive material layer, wherein the method forprocessing a photoresist-coated board further comprises a step ofvarying a duty ratio of the pulse-like laser beam in accordance with alength of a depressed portion or a projecting portion of the raised anddepressed pattern to be formed wherein when the length of the depressedportion or the projecting portion of the raised and depressed pattern isshorter than a predetermined length, the duty ratio of the pulse-likelaser beam is varied in accordance with the length of the depressedportion or the projecting portion of the raised and depressed pattern,and when the length of the depressed portion or the projecting portionof the raised and depressed pattern is equal to or longer than thepredetermined length, the duty ratio of the pulse-like laser beam iskept constant independently of the length of the depressed portion orthe projecting portion of the raised and depressed pattern, therebyexposing the photosensitive material layer to the laser beam.
 3. Amethod for manufacturing a stamper for a recording medium comprising astep of intermittently projecting a pulse-like laser beam onto aphotosensitive material layer on or above a light absorption layerprovided in the photoresist-coated board to expose the photosensitivematerial layer to the laser beam, and developing the photosensitivematerial layer, thereby forming a raised and depressed pattern on thephotosensitive material layer and a step for transferring the raised anddepressed pattern formed on the photosensitive material layer onto thestamper for a recording medium, thereby forming a raised and depressedpattern on the stamper for an optical recording medium wherein when alength of a depressed portion or a projecting portion of the raised anddepressed pattern is shorter than a predetermined length, a duty ratioof the pulse-like laser beam is varied in accordance with the length ofthe depressed portion or the projecting portion of the raised anddepressed pattern, and when the length of the depressed portion or theprojecting portion of the raised and depressed pattern is equal to orlonger than the predetermined length, the duty ratio of the pulse-likelaser beam is kept constant independently of the length of the depressedportion or the projecting portion of the raised and depressed pattern,thereby exposing the photosensitive material layer to the laser beam. 4.A method for manufacturing a stamper for a recording medium comprising astep of forming a raised and depressed pattern on a photosensitivematerial layer provided in a photoresist-coated board by intermittentlyprojecting a pulse-like laser beam onto the photosensitive materiallayer to expose the photosensitive material layer thereto and developingthe photosensitive material layer, and a step of transferring the raisedand depressed pattern formed on the photosensitive material layer onto astamper for a recording medium and forming a raised and depressedpattern on the stamper, wherein the method for manufacturing a stamperfor a recording medium further comprises a step of varying a duty ratioof the pulse-like laser beam in accordance with a length of a depressedportion or a projecting portion of the raised and depressed pattern tobe formed, wherein when the length of the depressed portion or theprojecting portion of the raised and depressed pattern is shorter than apredetermined length, the duty ratio of the pulse-like laser beam isvaried in accordance with the length of the depressed portion or theprojecting portion of the raised and depressed pattern, and when thelength of the depressed portion or the projecting portion of the raisedand depressed pattern is equal to or longer than the predeterminedlength, the duty ratio of the pulse-like laser beam is kept constantindependently of the length of the depressed portion or the projectingportion of the raised and depressed pattern, thereby exposing thephotosensitive material layer to the laser beam.
 5. A method formanufacturing a stamper for a recording medium comprising: forming araised and depressed pattern including a depressed portion or aprojecting portion corresponding to a pit on a photosensitive materiallayer on or above a light absorption layer provided in aphotoresist-coated board by projecting two or more pulses of a laserbeam onto the photosensitive material layer to expose the photosensitivematerial layer thereto and developing the photosensitive material layer;and transferring the raised and depressed pattern formed on thephotosensitive material layer onto a stamper for a recording medium andforming a raised and depressed pattern on the stamper, wherein a dutyratio of the pulse-like laser beam is varied in accordance with a lengthof a depressed portion or a projecting portion of the raised anddepressed pattern to be formed, and wherein when a length of a depressedportion or a projecting portion of the raised and depressed pattern isshorter than a predetermined length, the duty ratio of the pulse-likelaser beam is varied in accordance with the length of the depressedportion or the projecting portion of the raised and depressed patternand when a length of a depressed portion or a projecting portion of theraised and depressed pattern is equal to or longer than a predeterminedlength, the duty ratio of the pulse-like laser beam is kept constantindependently of the length of the depressed portion or the projectingportion of the raised and depressed pattern, thereby exposing thephotosensitive material layer to the laser beam.
 6. A method formanufacturing a recording medium comprising: forming a raised anddepressed pattern including a depressed portion or a projecting portioncorresponding to a pit on a photosensitive material layer on or above alight absorption layer provided in a photoresist-coated board byprojecting two or more pulses of a laser beam onto the photosensitivematerial layer to expose the photosensitive material layer thereto anddeveloping the photosensitive material laver; transferring the raisedand depressed pattern formed on the photosensitive material layer onto astamper for a recording medium and forming a raised and depressedpattern on the stamper; and transferring the raised and depressedpattern formed on the stamper onto a substrate for a recording medium toform pre-pits on the substrate. wherein a duty ratio of the pulse-likelaser beam is varied in accordance with a length of a depressed portionor a projecting portion of the raised and depressed pattern to beformed, and wherein when a length of a pre-pit is shorter than apredetermined length, the duty ratio of the pulse-like laser beam isvaried in accordance with the length of the pre-pit and when a length ofa pre-pit is equal to or longer than a predetermined length, the dutyratio of the laser beam is kept constant independently of the length ofthe pre-pit, thereby exposing the photosensitive material layer to thelaser beam.
 7. A method for manufacturing a recording medium inaccordance with claim 6 wherein the duty ratio of the pulse-like laserbeam is varied so that the photosensitive material layer is exposed tothe laser beam by setting the duty ratio of the pulse-like laser beam toa first value when a pre-pit having a first length is to be formed andthat the photosensitive material layer is exposed to the laser beam bysetting the duty ratio of the pulse-like laser beam to a second valuesmaller than the first value when a pre-pit having a second lengthlonger than the first length is to be formed.
 8. A method formanufacturing a recording medium comprising: forming a raised anddepressed pattern including a depressed portion or a projecting portioncorresponding to a pit on a photosensitive material layer provided in aphotoresist-coated board by projecting two or more pulses of a laserbeam onto the photosensitive material layer to expose the photosensitivematerial layer thereto and developing the photosensitive material layer;transferring the raised and depressed pattern formed on thephotosensitive material layer onto a stamper for an optical recordingmedium and forming a raised and depressed pattern on the stamper;transferring the raised and depressed pattern formed on the stamper ontoa substrate for a recording medium to form pre-pits on the substrate;and varying a duty ratio of the pulse-like laser beam in accordance witha length of a pre-pit to be formed, and wherein when a length of apre-pit is shorter than a predetermined length, the duty ratio of thepulse-like laser beam is varied in accordance with the length of thepre-pit and when a length of a pre-pit is equal to or longer than apredetermined length, the duty ratio of the pulse-like laser beam iskept constant independently of the length of the pre-pit, therebyexposing the photosensitive material layer to the laser beam.
 9. Amethod for manufacturing a recording medium in accordance with claim 8,wherein the duty ratio of the pulse-like laser beam is varied so thatthe photosensitive material layer is exposed to the laser beam bysetting the duty ratio of the pulse-like laser beam to a first valuewhen a pre-pit having a first length is to be formed and that thephotosensitive material layer is exposed to the laser beam by settingthe duty ratio of the pulse-like laser beam to a second value smallerthan the first value when a pre-pit having a second length longer thanthe first length is to be formed.